Photodetection of indicia on documents is well known. In such systems, a narrow beam of light is focused as a spot or slit of light on the relevant field to be scanned. The beam of light is moved over the field, either by the deflection of the beam or by movement of the document. Light reflected from, or transmitted through the document is focused on a photocell or other photodetector so that the indicia may be detected as variations of the photocell output. Typically, photocell outputs are small so that the photocell is connected to an amplifier and the amplifier output is further processed by the system.
Scanning indicia is complicated by the fact that the color or shade of the indicia may vary from job to job, changing the background or reference level against which the indicia must be detected. If the reference level is too low, extraneous marks on a document may be detected as indicia. If the reference level is too high, the system will miss parts of the indicia that are faded or lightly printed. In scanning indicia, such as bar codes or dash codes, on a moving document, it is critical that the reference level in an optical scanning system is optimally adjusted to read the scanned code accurately.
An optical scanning system for scanning dash codes is disclosed in U.S. Pat. No. 4,659,939, issued to John L. Lorenzo on Apr. 21, 1987 and assigned to the assignee of the present invention, which is incorporated herein by reference. The optical scanning system includes a potentiometer for adjustment to a sensitivity, or threshold, amplifier. However, dash code scanner sensitivity adjustment has proven to be a difficult task. A technician is needed to perform the adjustment which is a very subjective adjustment that relies on a trial and error method. Although a more precise sensitivity adjustment can be realized using an oscilloscope or other instruments, the adjustments are still subjective in nature and require trial and error.
It is an object of the present invention to provide a system and method which provides a precision sensitivity adjustment to an optical scanning system without the need for an oscilloscope or other instruments.
It is a further object of the present invention to provide a system and method which provides an automatic sensitivity adjustment to an optical scanning system.